Machines for plastic compounding have existed for many years now, and have progressed through many generations of development as technology improves and as plastic formulations change and place new demands on the compounding equipment. During this development, it has become technically and economically desirable to operate continuous processes.
For example, most production from linear low density polyethylene includes a continuous extruder machine. Polypropylene and EPDM, PVC (both rigid and flexible), thermoplastic rubbers, EVA, PE generally, and specialty formulations for video discs and records are other examples of compounding successes on continuous extruder machines.
The most effective continuous extruder designs are those which employ a twin screw extruder which is self cleaning and able to provide high capacity. The most efficient systems allow for multiple formulations to be processed on the same extruder, with the opportunity to customize the twin screw alignment for particular needs. Also used in some specific situations are single screw extruders, when the chemical and physical properties of the product require that treatment.
As is well known, chemical reactions in general and plastic production specifically needs to be controlled so that the reaction rate is maximized without adverse side effects. Thus heating and cooling functions are required, to maintain the plastics at a maximum efficiency. However, it is in this area that machines for continuous extrusion of plastics and the like have their most difficulties.
For example, polyethylene may be extruded at 200.degree. to 250.degree. F., while nylon is processed at about 650.degree. F. In both cases, excessive heat will cause degradation of the expected properties, while low heat will lead to longer reaction times or incomplete reactions. Each chemical system has its own needs, as some are strongly exothermic while others need heat to be added to drive the reaction. Sometimes, the fillers, coloring agents, flame retardants and the like call for special temperature considerations. Thus the extruder must meet a wide range of operating conditions if it is to be useful over any reasonable range of products.
Heating can be accomplished with electric heaters, steam or hot oil, for example, while cooling is done with water or air. It is also possible to control temperature by controlling the rate of the extruder screws, so that faster or slower rates enhance or retard exothermic or endothermic conditions as needed. Nevertheless, there presently does not exist an extruder system which can accommodate any significant part of the total market, particularly where different temperature conditions must be met for each varied chemistry.
Conventional extruder technology is not capable of accommodating the wide range of temperature conditions in a single machine. Of course, the requirements of any particular chemistry can be designed into a machine system, but the likelihood of that machine being usable for other systems is small. Thus it would be a great advance in the art if an extruder system could be provided which would operate over a wide range of temperature ranges, both for heating and cooling.
Presently, heater bands are wrapped around the segments of an extruder system, so that heat can be applied electrically at desired locations. These heaters provide a heat for melting, mixing and driving the reaction. Electric heat is effective in most systems but is limited by the specific design which is installed on any given unit. By this is meant that electric heating might be installed for a particular temperature and heat exchange range of conditions, but that unit might not be usable for other conditions.
Under these circumstances, additional heating is provided, usually by steam or hot oil. This form of heat requires that there be access to the region near the extruder path in order to be effective. To date, no effective system has been provide to accommodate this need.
Cooling in the present generation of extruder machines is done by a number of somewhat effective but not perfect designs. Single screw extruders are actually thick walled piping, and cooling is applied by wrapping a spiral coil around the pipe. Heat transfer is effected in a spiral path, but this has been found to be generally acceptable for single screw units.
Double screw extruders are much more complicated, of course, and spiral wrapping of cooling coils has not been nearly as effective in providing fast, direct cooling to the product as it is carried, mixed, and reacted by the twin screw design. In addition, twin screw or double screw extruders achieve substantially greater mixing per unit of length, so that both cooling and heating control needs to be faster and more precise.
Still, the only method used by the prior art is to wrap the double screw segments with heater bands. Cooling as also achieved by direct contact with the outside of the barrel segment. This does not present an even temperature profile to the mixing and extrusion zones. Thus mixing rates, reaction conditions and overall process efficiencies are limited more by the temperature control than by the capabilities of the extruder itself.
Accordingly, it is a primary object of this invention to provide an efficient and effective heating and cooling system for extruder devices, in which heating and cooling can be accomplished over the widest range possible, to permit much greater use of the extruder on a wider variety of formulations.
Other objects will appear herein.